The field of the invention relates generally to cargo inspection systems and, more particularly, to cargo inspection systems incorporating computed tomography (CT) and nuclear resonance fluorescence (NRF) to inspect cargo for contraband and methods for operating the same.
Since the events of Sep. 11, 2001, the Department of Homeland Security has increased security dramatically in U.S. airports. Such security efforts include screening passengers and carry-on bags and luggage for contraband including explosive materials.
Many of these systems employ single or few multi-view X-ray transmission technology. Although these systems enable the detection of weapons and blades, for example, they lack the capability of detecting explosives with a low false alarm rate.
CT provides a quantitative measure of material characteristics, regardless of location or the superposition of objects; a substantial advantage over conventional and multi-view X-ray transmission and radioisotope-based imaging systems. In a CT scanner, a large number of precise X-ray “views” are obtained at multiple angles. These views are then used to reconstruct planar or volumetric images. The image is a mapping of the X-ray mass attenuation value for each volume element (or voxel) within the imaged volume.
Systems employing CT are used widely in airports around the world on checked luggage to detect explosives that pose a threat to aviation safety. These systems employ an X-ray source and opposing detectors that rotate around a horizontal axis while the suitcase is translated along the same horizontal axis.
While such screening processes are reliable and suitable for break-bulk cargo, there is a need for inspecting large crates, pallets, and containers that are too large to inspect with conventional checked-luggage scanning systems. Further, it is too time consuming to remove and inspect the contents of each cargo container before loading the container for delivery to the destination. Only a portion of air cargo containers is inspected using currently available technologies including manual inspection, canine inspection, and/or trace detection. It is recognized that these inspection methods must be improved for automation and/or to obtain greater detection.
At least some known CT scanning systems are capable of detecting most explosives and other contraband. However, false alarms are occasionally raised due to similarities shared by explosives and other contraband and benign materials. There is a need for a system based on a different technology to clear most of the false alarms.
An imaging technique known as nuclear resonance fluorescence (NRF) is capable of producing a three-dimensional elemental image of an object by using a collimated beam of high-energy photons and an array of collimated detectors focused at all cargo depths. The high-energy photons cause nuclear states in the object elements to fluoresce. The identification of the elemental composition of an object is based on the characteristic energy of the gamma-ray energies re-emitted by the object and their intensities. The material identification is based on the elemental composition of the inspected object.
With least some known NRF scanning systems, inspection of large objects, such as cargo containers, would require a large period of time and a large number of detectors due to the intensity of currently available high-energy photon sources and the low fluorescence cross sections involved in NRF scanning. The increased time and equipment requirements increase the cost of a system. Systems using NRF scanning are more suitable for as secondary inspection systems after a primary inspection system indicates the presence of an item of interest.
Moreover, at least some known NRF scanning systems are further limited when used as secondary inspection systems due to insufficient information necessary for correcting for differential attenuation of the photon beam and the gamma rays emitted from the item of interest subjected to the photon beam. An inability to correct for the attenuation of this information may result in an inaccurate elemental composition determination.
Further, at least some known NRF scanning systems lack a shielding mechanism for the gamma-ray detectors. Because the location of an item of interest is unknown in such systems, a large number of detectors must be used. To include a shielding mechanism, such as an anti-Compton shield, for each detector would be cost-prohibitive. Moreover, because of the number of required detectors in such systems, space constraints preclude the inclusion of a shield for each detector. However, using fewer detectors requires that the NRF scanning configuration be optimized to enable the detectors to receive a maximum intensity of gamma rays emitted by the item of interest. Therefore, a system and/or method is required for optimizing the NRF scanning configuration.